Storage of biological and chemical samples is becoming widespread in the biotechnology and medical industries. To preserve many of these samples, the samples must be stored at or below freezing temperatures. Generally speaking, a regular freezer operates from −5° C. to −20° C., and an ultra-low temperature freezer operates from about −50° C. to about −90° C. (preferably at or about −80° C.) and a cryogenic freezer operates from about −140° C. to −196° C. (the boiling point of liquid nitrogen). The present invention is directed to ultra-low temperature freezers operating in the range of −50° C. to about −90° C., and preferably −80° C.
U.S. Pat. No. 6,941,762 to Felder et al., as well as U.S. Pat. Nos. 6,688,123; 6,581,395; and 6,467,287 also by Felder et al., describe various embodiments of an automated ultra-low temperature storage and retrieval system. In particular, these patents describe a system having a freezer compartment that is maintained at an ultra-low temperature from −50° C. to −90° C., preferably at about −80° C., under normal operating conditions. Storage racks are mounted within the insulated, ultra-low temperature freezer compartment. The storage racks can be mounted either in a fixed position or mounted to a rotating carousel. A mechanical robot is provided within the ultra-low temperature storage compartment to place sample storage containers in the storage racks and retrieve the storage containers from the racks. The sample storage containers are typically SBS footprint compatible, and take the form of microtiter plates, tube storage racks, reservoirs or other SBS format containers. The robot also communicates with an access module in order to introduce the sample storage containers into the system and retrieve the containers for use outside of the system. The Felder et al. patents describe the use of a climate control chamber which uses a dry gas purge to reduce the humidity in the access module. It is typical to locate the drive motors outside of the freezer compartment, not only because the motors have difficulty operating at ultra-low temperatures, but also to reduce heat generation within the ultra-low temperature storage compartment.
These ultra-low temperature storage and retrieval systems have a capacity of several hundred or more sample storage containers, such as microtiter plates or tube storage racks. Although there are a wide variety of manufacturers for freezer systems that are capable of cooling the storage compartment to an ultra-low temperature of, for example −80° C., the cooling process with some freezer systems is not particularly efficient. Normally, it takes about 24 hours to cool the freezer compartment to −80° C. in preparation for loading the system with biological or chemical samples.
It has been found that many biological samples stored in ultra-low temperature systems are often contained in sealed plastic laboratory tubes held in tube storage containers in arrays of, for example, 48 or 96 tubes. In some cases, a two-dimensional barcode is adhered to the bottom of the tubes that is able to be read through the bottom of the storage containers. In other cases, a one-dimensional bar code is placed on the sidewall of the tube. In either case, bar coding facilitates data entry into the control system which keeps track of the location of each of the biological samples. It is also typical for the sample storage containers themselves to have a barcode.
In these ultra-low temperature storage and retrieval systems, it is desirable to reduce the accumulation of frost within the ultra-low temperature freezer compartment. Excessive frost can cause the system, and in particular the robot and the other components of the retrieval mechanism, to malfunction. Therefore, it is necessary to defrost the systems on a fairly regular basis. The defrosting procedure, however, is normally time-consuming. Typically, all of the sample storage containers must be transferred to a separate ultra-low temperature freezer, and then after defrosting and recooling of the system, reintroduced on a one-by-one basis. Not only is the defrosting procedure quite time-consuming, but it can also lead to premature wear of system components, for example, robotic bearings or other components. One object of the present invention is to reduce the amount of moisture ingress allowed into the ultra-low temperature freezer compartment during normal operation, in order to reduce the need to defrost as often as is now typical.
While a significant amount of moisture in current day systems is allowed into the ultra-low temperature freezer compartment through the access module when sample storage containers are introduced or retrieved, moisture and heat can also leak into the insulated, ultra-low temperature freezer compartment at any location where the freezer wall is breached. For example, openings to pass a mechanical drive shaft through the freezer wall even if the opening is sealed can provide an opportunity for leakage, especially after the seal is worn.
To commercially manufacture the system disclosed in Felder et al., a custom designed freezer body was used to house the storage racks and storage carousel, and the robot and its drive mechanism, as well as accommodate the access module and drive motors. Certain components such as the carousel and the racks, as well as supports for the robot, are mounted directly to the inner wall of the freezer compartment. This can lead to distortion problems during installation because of material shrinkage due to the ultra-low temperatures. It should be noted that the placement of the reach arm or interchange mechanism must be accurate, especially with respect to the rotational accuracy, otherwise the system may malfunction and could possibly cause loss of samples. Therefore, it has not been uncommon for technicians to spend significant time and effort accounting for thermal distortions during system set-up.
Also, referring to the system disclosed in Felder et al, the robot is mounted on a cylindrical base which is mounted through the floor of the freezer compartment. The robot motors are mounted to the cylindrical base outside of the freezer compartment. The cylindrical base, as well as substantially the entire robot, are rotated in order to position the reach arm. However, this design requires an active seal between the cylindrical base and the floor of the freezer which can at times be somewhat difficult to achieve and can become a source of wear. In a similar fashion, the motor for driving the storage rack carousel is mounted below the floor of the freezer and its drive shaft must penetrate the wall of the floor of the freezer in order to drive the carousel. Again, although the penetrating drive shaft is sealed, the breaching of the freezer wall provides an opportunity for heat and/or moisture to leak into the ultra-low temperature freezer compartment.
An issue also arises when it is desired to retrieve less than all of the storage tubes from a stored sample container, which is more often the case than not in these applications. It is not desirable to remove the entire container from the system. The removal procedure allows for the ingress of moisture in to the ultra-low temperature storage compartment, and also threatens that the other samples held in the same container will be thawed at least partially when removed from the system even if temporarily. While tube picking mechanisms are generally known in the art, the environment within the ultra-low temperature freezer compartment is typically too cold to ensure reliable operation of conventional tube picking mechanisms.